Open source router software
I installed dd-wrt on my WRT610n. I am pretty impressed! I basically have a server connected to the Internet that I can access from anywhere in the world. I can do ftp, VPN, start and stop PCs on my home network. All besides being a wireless/wired router of course.
It is amazing how many things I can think about, like having a virtual distributed file system on the Net, for example, cool!